Abstract: China is rich in marine resources, and with the development of its economy and the improvement of its transportation level, the use of immersed tunnel technology is increasingly more extensive. The Shenzhen–Zhongshan Bridge is the first steel shell immersed tunnel in China. The immersed tunnel is located in a sea mud area that is not easy to inspect and maintain, and its steel shell structure is eroded by seawater, which shortens its service cycle, and severe corrosion affects its safe operation. Its durability requirement is 100 years. For the service environment and ultrahigh durability requirements of the Shenzhen–Zhongshan Bridge steel shell concrete immersed pipe and many other characteristics, at present, few engineering and research references at home and abroad can be used. Thus, the corrosion development law of the outer wall of an immersed steel shell in a marine environment must be studied and revealed. In this work, dissolved oxygen (15.2 mg·L^–1) was artificially added to a simulated seawater solution as a depolarizing agent to realize the acceleration process of a corrosion simulation acceleration test in the laboratory. The test set cycle was 1, 7, 15, 30, 90, 180, and 365 d, and the test temperature was 25 ℃. Through the electrochemical impedance spectroscopy (EIS), the Tafel polarization curve, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), confocal laser scanning microscopy (CLSM), and other analytical and testing methods of samples with different test cycles, the corrosion occurrence and development law of Q390C low-alloy high-strength structural steel used in deep–medium channel immersed tunnel steel shells under simulated seawater conditions was studied. The corrosion products of Q390C steel in seawater are mainly found to be Fe₃O₄, α-FeOOH, γ-FeOOH, and a small amount of CaCO₃, and their uniform corrosion and local corrosion rates decrease exponentially and eventually tend to stabilize. CLSM test shows that the surface of the specimen begins to corrode uniformly after a test cycle of 15 d, and the pitting corrosion pit depth of the specimen with a test cycle of 365 d can reach 99 μm. The long-term accelerated corrosion test of the steel shell of an immersed tunnel in seawater in this paper is of great importance to ensure the long-life durability of immersed tunnels in marine engineering and similar construction projects.